The present disclosure relates to a reading module used in a digital copier, an image scanner, or the like, for reading image light reflected from a document illuminated by light, and to an image reading device and an image forming apparatus including the reading module.
Conventionally, there are two types of reading systems for the image reading device mounted in a multifunction peripheral or the like using an electrophotographic process; one is a CCD system using a charge coupled element called a charge coupled device (CCD) sensor, and the other is a CIS system using a photoelectric conversion element called a complementary MOS (CMOS) sensor.
The CCD system is a system in which a reduced image is formed on an image sensor having a size of ⅕ to 1/9 of a document size by using a plurality of flat mirrors and optical lenses so as to read the image. The CCD system has a merit of a large depth of field. Here, the depth of field is a range in which a subject (i.e., the document) looks focused even if it is shifted in an optical axis direction from a just focus position. In other words, a large depth of field means that even if the document is shifted a little from a prescribed position, a not so blurred image can be read.
On the other hand, the CCD system has a demerit of a very long optical path length (a distance of light propagation from a subject to a sensor), which is 200 to 500 mm. In order to secure this optical path length in a limited space of a carriage, the image reading device uses a plurality of flat mirrors to change the propagation direction of the light. As a result, the number of components is increased, and cost is increased. In addition, when using a lens in the optical system, a difference of refractive index due to wavelength causes color aberration. In order to correct this color aberration, a plurality of lenses is necessary. Thus, use of a plurality of lenses also causes an increase of cost.
In the CIS system, a plurality of erect equal magnification rod lenses are arranged in an array, and the image is formed on an image sensor having the same size as a document. The CIS system has a merit of a relatively short optical path length of 10 mm to 20 mm and a compact size compared with the CCD system. In addition, because only the rod lenses are used for forming an image, mirrors are not necessary though they are necessary in the CCD system, and hence a scanner unit including a CIS sensor can be slimmer. In addition, because it has a simple structure, low cost can be achieved. On the other hand, because the CIS system has a very small depth of field, when a document is shifted in the optical axis direction from a prescribed position, influence of blur due to image bleeding becomes large by each lens magnification shift. As a result, the CIS system has a demerit that a book document or a document having an uneven surface cannot be uniformly read.
In recent years, there are proposed image reading systems using a reflecting mirror array in the imaging optical system in a different manner from the CCD system or the CIS system described above. In this system, reflecting mirrors are arranged in an array, and a reduced inverted image of a document is formed on a sensor for each corresponding reading area. However, unlike the CIS system using a rod lens array, one area is read to form an image by one optical system. In addition, a telecentric system is adopted as an image forming system, and there is no occurrence of image bleeding due to overlapping of images having different magnifications so that image blur is suppressed, when reading the document separately for each of the areas. Thus, a compound eye reading system is realized.
Further, in this system, only mirrors are used in the optical system, and hence color aberration does not occur unlike a case where a lens is used in the optical system. Therefore, it is not necessary to perform a correction concerning color aberration, and hence the number of elements constituting the optical system can be reduced.